Various methods are known for delivering and coupling acoustic energy to a region of tissue to perform diagnostic and/or therapeutic and/or cosmetic procedures on a patient's tissue. Among such procedures are for example, non-invasive assaying of blood analytes, drug delivery by phonophoresis, lithotripsy, tissue ablation and lysis of fat cells for cosmetic removal of adipose tissue.
U.S. Pat. No. 5,267,985 describes “diffusion of a substance to a local area of material or tissue by providing ultrasonic energy to the substance and material in two or more distinct frequencies simultaneously. Each of the distinct frequencies of ultrasonic energy is chosen to enhance permeability of the substance through one or more diffusion rate-limiting sections of the material.” The patent notes that the intensities of the different frequencies of ultrasound “typically do not coincide and, as a result, the use of a multiple frequency system can provide a more uniform axial intensity over a greater distance than can a single frequency ultrasound system.” In an embodiment of the invention, the acoustic energy is produced by “driving a single transducer with voltage corresponding to the sum of voltages required to produce each frequency desired”.
For many types of therapeutic and/or cosmetic acoustic applications, such as for example lithotripsy, tissue ablation and lysis noted above, sufficient acoustic energy must be delivered to a tissue region to destroy and remove tissue in the region. Generally, the acoustic energy is delivered by focusing at least one beam of relatively intense ultrasound on the region. The high intensity, focused ultrasound, conventionally referred to by the acronym “HIFU”, generates thermal stress and/or cavitation that disrupts and destroys the tissue. Tissue raised to and maintained at a temperature above about 42° C. rapidly dies and mechanical stresses generated by cavitation breach and tear cell membranes of the tissue.
Various studies have indicated that efficacy of destruction and removal of tissue from a tissue region using high intensity ultrasound can generally be enhanced by applying more than one frequency of ultrasound to the region. Typically, the ultrasound is applied to the region by generating two beams of different frequency ultrasound using separate transducers and driving circuits and focusing the beams on the region.
P. Z. He et al in “Dual-Frequency High Intensity Focused Ultrasound (HIFU) Accelerating Therapy”; Proceedings of the IEEE Engineering in Medicine and Biology, 27th Annual Conference, Shanghai, China; Sep. 1-4, 2005; pp 213-216, concluded from “preliminary experimental results” that “dual frequency HIFU induces larger lesion in tissue than conventional single frequency HIFU under the same exposure conditions”. The dual frequency experiments were carried out by irradiating tissue regions, apparently simultaneously, with ultrasound at 1.563 MHz and 1.573 MHz radiated by central disc and confocal annulus PZT-4 transducers respectively. G. Iernetti et al in “Enhancement of high-frequency cavitation effects by a low frequency stimulation”, Ultrasonics Sonochemistry 4 (1997) pp 263-268, describe enhancing cavitation effects in tissue generated by relatively high frequency ultrasound at 700 kHz using relatively low frequency ultrasound at 20 kHz. The low frequency ultrasound was used to generate a “stimulating field” that was applied to a tissue region to amplify cavitation effects of the high frequency ultrasound at different stages of cavitation in the tissue region caused by the high frequency ultrasound.
All the above referenced documents are incorporated herein by reference.